Return nothing if no changes
Return nothing if no changes

beforeUnload should return nothing if we don’t have unsaved changes. It had returned null which IE11 thinks is a value. Redmine #5074

ENCODE-DCC/encoded,T2DREAM/t2dream-portal,T2DREAM/t2dream-portal,ENCODE-DCC/encoded,ENCODE-DCC/encoded,ENCODE-DCC/encoded,T2DREAM/t2dream-portal,T2DREAM/t2dream-portal